Modulating system



May 2, 1950 G. H. BROWN 2,506,132

' MODULATING SYSTEM Filed June 30, 1948 INVENTOR GEORGE HBROMV am May 2, 1950' UNITED STATE MODULATING sYs'rEM 4 George Ii. Brown, Princeton, N. J.,,'asaignor to' of America, a corporation Radio Corporation of Delaware Application June 30, 1948;801'IQINO. 38,249

This invention relates to systems for modulating radio frequency carrier energy, and more particularly to improvements in absorption modulators.

One of the principal objects of the invention is 6 Claims, (Oi. ass-4s) to provide absorption modulators capable of producing pure amplitude modulation of a radio frequency carrier, wherein phase modulation is bal anced out. v

Another important object is to provide modulators of the described type which may be designed to present input and output impedances which do not vary during a modulation cycle.

- A further object of the present invention is to provide improved modulators of the mechanical.

' the modulator to be described.

' Figure 2 is a schematic diagram of a modulator; system including two networks similar to that of Figure 1 and arranged so that phase variations produced by one are compensated by equal and opposite phase variations introduced by the other, and

Figure 3 shows a coaxial line section used as a reactance transformer in a manner applicable to 'the system of Figure 2.

In the circuit shown in Figure 1, a radio frequen1cy source I is connected through transmission lines 3 and 5 to resistive devices such as resistors 1 and 9, respectively. The lines 3 and}, as well as all other transmission lines in Figures 1 and 2 of the drawing, are represented as single conductors. It is to be understood that said lines may be and preferably are coaxial cables, with their outer conductors grounded and not shown in the drawing. However, the following description will apply as well to systems using single wire conductors adjacent a. conductive. ground plane, or two conductor parallel wire transmission lines.

The resistors l and 9 are equal, and the lines 3 and 5 are designed to have a characteristic impedance Z==R, where R is the resistance of each resistor. The lines 3 and 5 each have a length of one quarter wavelength, or an odd number of quarter wavelengths, at the frequency of the source i. An impedance element i3 is connected in parallel with the resistor 1, and a second impedance element It is connected in parallel with the resistor 8. Denoting the impedance of the 2 element l3 as m and that or the element II as ,Zz, said elements are designed or adjusted so It can be demonstrated that the impedance presented by the above-described network to the source i is Zc and is constant irrespective of the actual impedances of the elements l3 and I5, as long as the foregoing relationship is satisfied. Assuming the voltage at the source to have a constant amplitude Em, the amplitude E1 of the voltage across the resistor I will depend on the impedance Z1, being zero when Z1 is zero and equal to E111 when Z1 is infinite. The voltage E2 across the resistor 9 will also vary, in accordance with the corresponding variations of Z2.

Thus, by varying the impedances of both elements l3 and I5 together and in inverse manner, the energy reaching the resistor I may be modulated in amplitude without reflecting any variation in load on the source I. If the impedances Z1 and Z2 were both pure resistances at all times, the modulation would be purely in ampiitude, the phase of the carrier signal at the re sister I remaining constant with respect to that at the source i. However, it is diflicult in practice to produce a radio frequency resistance varying according to a. predetermined law, owing to the fact that resistance devices have inductance and/or capacitance which must be compensated.

Z1 and Z2 may be substantially pure reactances, varying as described above. It will be noted that when Z1 is a positive (i. e. inductive) reactance, Z: must be capacitive, and vice versa. The required variable reactances may be provided by transmission line sections including movable shorting plugs or plungers which periodically vary the-effective lengths of the line sections to vary the reactances they present.

For mechanical reasons, and particularly to avoid the use of sliding contacts, it is preferred at present to make the variable portions of the reactance elements in the form of variable capacitors. The varying capacitive reactahce is transformed by means of a transmission line network to a reactance which varies in the required manner. 1

Referring to Figure 3, a variable capacitor 2i is, connected across one end of a quarter Wavelength line section I9. An adjustable line stub 23 is connected in parallel with the capacitor 2|, and a second adjustable stub 25 is connected acrossthe other end of the quarter wave section I8. With the capacitor 2! set to provide its minimum capacitance (maximum reactance), the stub 23 is adjusted to provide an inductive reactance substantially equal to the capacitive reactance. The two reactances resonate to provide substantially an open circuit across the right hand end of the line l9. Owing to the impedance inversion characteristic of the quarter wave line, the high impedance at the right hand end 01' the line appears as a relatively very low impedance at the left hand end of the line.

The capacitor 2| is then set to provide its maximum capacitance. Since the capacitive reactance is now relatively low at the right hand end of the line, the high inductive reactance of the stub 23 has substantially no eflect. The low capacitive reactance is inverted by the line l9 and appears as a. relatively high inductive reactance at the left hand end of the line. The stub 25 is adjusted to provide a capacitive reactance substantially equal to this inductive reactance. The two reactances resonate to provide a very high impedance at the left hand end of the line l9.

As the capacitance of the capacitor 2| is varied from its minimum to its maximum value, the impedance appearing at the left hand end of the line 19 varies from a relatively low value to an extremely high value. Preferably the stubs 23 and 25 are adjusted so as to provide in complete compensation of the reactances, so that the efiective reactance at the left hand end of the line varies between a relatively low capacitive reactance and a relatively high inductive reactance.

The reactance element l3 in the network of Figure 1 may be a circuit like that shown in Figure 3. The element l5 may be similar but include a further quarter wavelength line section connected between the line [9 and the resistor 9. With this arrangement both reactance elements 13 and I5 may be varied in identical fashion. Supposing the reactances presented at a given instant by the variable reactance circuits in the elements l3 and I5 have a value jX; the reactance presented by the element l3 across the resistor I will be y'X. However, the reactance presented by the element 15 through its quarter wavelength line to the resistor 9 will be:

X @=;1 ar

jX X Thus the reactances applied to the resistors and 9 will fulfill the relationship required for attaining a constant impedance 20 at the point of connection of the source i.

As the capacitors 2| in the impedance elements l3 and I5 are varied together, the energy reaching the resistor 1 is modulated in amplitude without reflecting any variation in load on the source. However, the voltage E1 at the resistor I will vary in phase, aproaching a lag of 90 degrees with respect to the voltage Em as the reactance of the element l3 approaches infinity, and approaching a lag of zero with respect to Em as the reactance approaches zero. Figure 1 when used with variable reactances as an amplitude modulator will also introduce undesired phase modulation.

Referring to Figure 2, the resistor l of Figure 1 is replaced by an additional network, similar to the network of Figure 1. The additional quarter wave line section associated with the reactance element i5 is shown in Figure 2 as the line H. The various elements of the second net work are designated by the reference characters applied to the corresponding elements of the first Consequently, the circuit of' network, but primed The first network acts as a carrier source for the second network. and is connected thereto through a transmission line I! of any convenient length. The load 1' of the second network is represented by a block which is intended to signify any useful load, for example, an antenna. The second network may be substantially identical with the first, except that the reactances of the reactance elements it and I! are equal and opposite to those of the elements l3 and I5. Thus if the reactance X of the element I3 is inductive, the reactance -X 01' the element I3 is capacitive. The reactance elements [3' and may be structurally the same as the elements I3 and IE, but be varied diilerently to maintain the required relationships of sign and magnitude throughout the modulation cycle.

Now suppose all four reactances are varied simultaneously so that they remain equal in magnitude and maintain the above-described sign relationship. The first power dividing network will act as described in connection with Figure 1, producing amplitude modulation and also phase modulation. The second network will accept the resulting modulated signal and modulate it still further; the amplitude modulation will be in phase with that produced by the first network,

' because the reactances X and X of the two networks are varied together. Thus, the total amplitude modulation will be the square of that provided by either network.

The phase modulation introduced by the second network will be opposite to that of the first, because the signs of the reactances are opposite, making the phase shift occur between the limits of zero and degrees lead instead of zero and 90 degrees lag. Thus the output to the load 1' will be purely amplitude modulated.

The invention has been described as an improvement in absorption modulators, wherein the available input power is divided between a fixed load and fixed resistors, by variations in substantially pure reactance. elements connected thereto. The use of variable reactance elements instead of variable resistors avoids the problems of compensating the variable stray reactances which are characteristic of variable resistors at radio frequencies. The variable phase shifts which are introduced by reactances in one part of the system are cancelled by opposite phase shifts introduced similarly in another part of the system, so that substantially pure amplitude modulation is provided.

I claim as my invention:

1. In a system of the described type, a source of carrier energy to be modulated, a power division network connected to said source, said network including two output points and a pair of reactance elements whose reactances determine the apportionment of said carrier energy between said two output points, and means varying said reactance elements simultaneously according to the modulation to be produced, thereby varying the power available at said output points but also introducing corresponding phase variations at said points; a resistive device connected to one oi said points, and a second power division network connected to the other of said output points as a source, said second network being similar to said first, a resistive device connected to one of the output points of said second network, and a load connected to the other output point of said second network, and means varying the reactance elements of said second network to present reactances :thereto which are the negatives ofthe corresponding reactances presented by the reactance elements of said first network, whereby the carrier energy reaching said load is constant in phase with respect to that supplied by said source.

2. The invention set forth in claim 1, wherein connected to the end of said second branch line, said first and second reactance elements being identical, the reactances of said third and fourth each of said power division networks comprises of said elements includes a quarter wavelength line connected to invert the reactance thereof, and said two reactance elements in said second network are substantially identical with each other except that one includes a quarter wavelength line connected to invert the. reactance thereof.

5. In a system of the described type, a source of carrier energy to be modulated in amplitude and a load to be supplied with said modulated energy, a main transmission line connecting said source to said load, a branch line connected to said main line adjacent said source and having a length of one half wavelength degrees at the frequency of said carrier, a resistor connected to the midpoint of said branch line. and a reactance element connected to the end of said branch line, a second reactance element connected to said main line one quarter wavelength in thedirection of said load from the point of connection of said branch line, a third reactance element connected to said main line adjacent said load, a

second branch line having a length of one half wave length connected to said main line at a point one quarter wavelength in the direction of said source from said third reactance element,

a resistor connected to the midpoint of said second branch line and a fourth reactance element elements being equal in magnitude and opposite in sign to said first and second reactance elements, and means varying the I magnitudes of the reactances of all of said elements identically in accordance with the modulation to-be produced.

6. In a system of the described type, an input circuit for receiving carrier energy to be modulated, a power division network connected to said input circuit, said network including two output points and a pair of reactance elements whose reactances determine the apportionment of said carrier energy between said two output points, and means varying said reactance elements simultaneously according to the modulatlon to be produced, thereby varying the power available at said output points but also introducing corresponding phase variations at said points; a resistive device connected to one of said points, and a second power division network connected to the other of said output points as a source, said second network being similar to said first, a resistive device connected to one of the output points of said second network, means for connecting a load to the other output point of said second network, and means varying the reactance elements of said second network to present reactances thereto which are the neganetwork is constant in phase with that supplied at said input circuit.

GEORGE H. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,198,025 Davies et al Apr. 23, 1940 2,350,552 George et al June 6, 1944 2,368,693 Watts Feb. 6, 1945 

